That is why the law specifically references that the firearms in question must not pass state lines, and must be inscribed with "Fabricated in _____".
They are trying to get a case before SCOTUS to overturn the HORRIBLE Wickard v. Filburn case from the 30's. Its about time.
It'll never fly, though. Even SCOTUS is too wrapped in federal power to strip away their ability to regulate everything. The Wickard v Filburn case is what allows them to control schools, traffic, highways, federal funding handouts, gun laws, gun free school zone act, controlled substances, and just about every other thing that the feds do beyond their constitutional limits. Reversing that decision would shrink the federal government (and power) a good 50% or more overnight. They'll never do it, since they've all been at the federal tit too long themselves.
Ever since then (and the opening of the Fed Bank), our country has been doomed, it is just a matter of how long it takes for tyranny to grow.